Heat-curable resins, which are being widely used as leading-edge high-performance material and the like, are employed as polymer materials excellent in heat resistance, environmental stability, dynamic properties and electrical properties in applications such as protective film or electrical insulating material such as a solder resist or an interlayer insulator; a sealing material for an IC or a very large scale integrated circuit; and a laminate sheet.
However, when used for a solder resist, there is a problem of warpage caused by large shrinkages occurring during curing process and during cooling process after the curing of the heat-curable resin.
An example of a conventional heat-curable resist includes such an epoxy resin-based resist composition containing an epoxy resin and a dibasic acid anhydride as essential ingredients as disclosed in JP-B-05-75032. However, the conventional heat-curable resist involves a problem in that resistance to moisture and heat, soldering heat resistance and in particular, resistance to tin plating are deteriorated, for example in a case where the resist is formed into a coating film imparted with low-warpage property and flexibility.
JP-A-11-158252 discloses a composition for forming a flexible protective film with low-warpage property. The composition contains a polyfunctional epoxy resin and a specific polyacrylic resin as essential ingredients. Therefore, in order to balance these components with properties competing against them (such as plating resistance) in the composition, the polyacrylic resin must be regulated in terms of both an acid value and a glass transition temperature. In addition, plating resistance will not always be sufficient if low-warpage property is to be obtained.